Carbon dioxide refrigeration systems

ABSTRACT

Refrigeration apparatus utilizing solid carbon dioxide as a refrigerant comprising a pair of refrigeration compartments, a solid carbon dioxide hopper defined by a pair of hollow condensers arranged in the shape of a V for a circulating heatexchange fluid. A pair of evaporators vertically below the hopper and defining a chimney between the compartments for the circulation of air being cooled. An air cargo container having a lower hopper and an upper evaporator. In the conduit connecting the condenser at the hopper to the evaporator there is means for providing bubbles to lift the heat exchanger liquid to the evaporator.

7/1951 Storm..........................62/l66 [54] CARBON DIOXIDE REFRIGERATION 4/1953 Woods.........................62/l66 SYSTEMS [72] Inventors: Emmett P. Glynn,

Lamont; award Primary Examiner Meyer Perlin L. HS Ch' both f In.

ma 0 Attorney-Anderson, Luedeka, Fitch, Even and Tabin Corporation,

[73] Assignee: Liquid Carbonic Chicago, Ill.

[57] ABSTRACT Refrigeration apparatus utilizing solid carbon dioxide as a refrigerant comprising a pair of refrigeration com- [22] Filed: Aug. 26, 1970 [21] Appl. No.: 66,961

partments, a solid carbon dioxide hopper defined by a pair of hollow condensers arranged in the shape of a V for a circulating heat-exchange fluid. A pair of [52] US. 62/384 [51] Int. 3/12 .62/165, 166, 167, 168, 384,

evaporators vertically below the hopper and defining a chimney between the compartments for the circula- [58] Field of Search.......

tion of air being cooled. An air cargo container having a lower hopper and an upper evaporator. In the conduit connecting the condenser at the hopper to the [56] References Cited UNITED STATES PATENTS evaporator there is means for providing bubbles to lift the heat exchanger liquid to the evaporator.

Strobell ..................;....62/166 Brown 7 Claims, 7 Drawing Figures PATENTED "N 3 i972 IIPIIIIIIH "H" H h-"d I I LI HHHNI P CARBON DIOXIDE REFRIGERATION SYSTEMS This invention relates to cooling systems, and more particularly to cooling systems which are particularly suited for employing solid carbon dioxide as a refrigerant.

Solid carbon dioxide is an excellent expendable refrigerant because it is relatively inexpensive and has a very high cooling capacity per unit of weight and volume. Because of its relatively low freezing point (-l09.6 F.), there are advantages to using carbon dioxide in an indirect refrigeration system rather than employing it to directly cool particular products. As a result, systems have been developed using a solid carbon dioxide to cool an intermediate heat-exchange liquid, for example, methylene chloride, having a low freezing point which in turn is used to absorb heat from the product being refrigerated. Low-freezing liquid solutions of relatively high percentages of methanol in water have also been employed as heat-exchange liquids. Systems using other subcooled intermediate liquid eutectics of relatively high heat capacities have been used fairly extensively in truck refrigeration and have also found applications in railroad car refrigeration. Improved cooling systems employing such intermediate heat-exchange liquids which are particularly adapted for use with solid carbon dioxide are still desired.

It is an object of the present invention to provide an improved cooling system using an intermediate heatexchange fluid that is particularly designed to employ solid carbon dioxide as the refrigeration source. Another object of the invention is to provide cooling systems for effectively controlling the releaseof the inherent refrigeration value of solid carbon dioxide, which systems are simple in construction and contain few moving parts, thereby assuring good reliability in operation. A further object of the invention is to provide cooling systems employing solid carbon dioxide which are particularly adaptable for use in portable apparatus and which have the capability to operate with substantial independence of environmental conditions.

These and other objects of the invention will be apparent from the following detailed description of several systems embodying various features of the invention when read in conjunction with the accompanying drawings wherein:

FIG. 1 is a front elevation view, with portions broken away, of apparatus incorporating a cooling system designed for operation with solid carbon dioxide and embodying various features of the invention;

FIG. 2 is an enlarged sectional view of the apparatus shown in FIG. 1 taken generally along the lines 22;

FIG. 3 is a front elevational view of another apparatus employing a cooling system also embodying various features of the invention taken along 'line 33 of FIG. 4;

FIG. 4 is a sectional view taken generally along line 4-4 of FIG. 3;

FIG. 5 is an enlarged fragmentary view FIG-4; and

FIGS. 6 and 7 are views similar to FIG. 5 of altemative embodiments which may be used.

Shown in FIGS. 1 and 2 is a refrigeration apparatus 11 in the form of a cart having a chassis or enclosure 13 which is mounted on four wheels 15 to make it easily transportable. A pair of refrigeration compartments 17 are arranged generally side-by-side in the cart. The walls of the chassis are suitably insulated, and access to each compartment 17 is provided by a suitably insulated, vertically hinged door 19. The illustrated apparatus 11 is designed for food service, and each compartment 17 has a plurality of cantilever brackets 21 mounted therewithin on the respective outer walls. The brackets 21 are used to support food trays at spaced vertical locations within the compartments 17.

A V-shaped hopper 23 is arranged centrally of the upper portion of the chassis 13, and access to the hopper 23 for supplying solid carbon dioxide is provided by an upper hinged door 25. The hopper 23 is formed by a pair of hollow condenser plates 29 arranged at an acute angle to each other. The V-shape of the hopper 23 has been found to assure that good heat transfer is achieved between discrete carbon dioxide nuggets (with which the hopper 23 is preferably filled) and the fluid in the condenser plates 27 which form the bottom walls of the hopper. Nuggets are short cylinders of dense solid carbon dioxide that can be handled like other particulate materials. The undersurface of the V- shaped hopper 23 also serves to smoothly direct the circulation of air either entering or leaving the refrigeration compartments 17.

The two refrigeration compartments 17 are spaced apart to provide a vertical chimney 29 therebetween, which chimney is generally defined by a pair of vertical evaporator plates 31. These two evaporator plates 31 also respectively define the inner boundary of each refrigeration compartment 17 and as such constitute the inner sidewall of each compartment. The lower end of each of the evaporators 31 is spaced a given distance above the bottom of the chassis 13 to provide lower circulation openings 33. Likewise, the upper end of each evaporator 31 is spaced a desired distance below the inclined undersurface of the hopper 23 to provide upper openings 34. The upper and lower openings 35 and 33 at the top and the bottom of the evaporator plates 31 facilitate the circulation of air through the chimney 29 and the two refrigeration compartments 17.

Heat transfer to the cold evaporator plates 31 cools the air adjacent both surfaces of each of the evaporator plates. The more dense, cooler air gravitates downward in the chimney 29 and flows out through the lower openings 33 into the bottom of the refrigeration com partments 17. When the food or other material in the compartments 17 is at the desired temperature, warming of the air is via heat flow into the cart 11 through the outer walls of the chassis 13. Such heat input warms the air adjacent the inner wall surfaces of the refrigeration compartments l7 and causes an uplift of the less dense air adjacent these locations, thus establishing a natural convection flow upward along the walls of the refrigeration compartment 17. Overall circulation is achieved as this warmer air at the top of the compartments 17 is drawn through the upper openings 35 into the chimney 29 and downward along the evaporator plates 31 to replace the descending cooled column of air.

ln many cases this natural convention flow will be sufficient to accomplish a rapid enough cool-down of the apparatus initially and to maintain sufficiently uniform temperatures throughout the refrigeration compartments 17. However, in cases where natural convection of the air is not considered sufiicient, forced convection of the air in the cart 11 is employed. Illustrated is a blower 37 having a self-contained, battery-powered motor. It has surprisingly been found that better refrigeration performance is obtained when the blower 37 is operated in a manner which reverses the downward direction of air in the chimney and causes the air to move upward, than if the blower were used to merely assist the natural convection. Performance is improved both from the standpoint of more rapid cooldown and from the standpoint of more uniformity of temperature throughout the refrigeration compartments' l7. Normally, the .upper portions of the refrigeration compartments are more difficult to maintain in the same narrow temperature range with the lower portions thereof. It is believed that the splitting of the upward moving column of air by the undersurfaces of the inclined walls of the hopper 23 at the top of the chimney 29 causes an effective spreading of the cool air throughout the upper portions of the refrigeration compartments 17.

More particularly, the apparatus employs a completely closed system of an intermediate heatexchange liquid, preferably a Freon. Freons, or polyhalogenated flurocarbons as they are sometimes called, are short chain paraflin hydrocarbons which have been substituted with one atom of fluorine and at least another atom of halogen, which may be fluorine, chlorine or bromine. Freons are well known for their use as heat-exchange fluids, and a particular Freon which is generally chosen having physical properties which best complement the overall characteristics of the system, including the desired temperature range to be maintained in the refrigeration compartment 17 and the temperature of the refrigerant source. In general, when solid carbon dioxide is used as the source of refrigeration and it is desirable to maintain a temperature in the neighborhood of 40 F. in the refrigeration compartment 17, the preferred freons are Freon 12, Freon 21, and Freon ll.

In the illustrated apparatus 11, the condenser plates 27 which are disposed in the V-shape arrangement extend from the front wall 39 to the rear wall 41 of the chassis 13 and constitute the hopper 23. If desired to increase its volumetric capacity, the upper portions of the condensers might be made vertical and not substantially detract from the effect gained by having the lower surfaces inclined. The condensers 27 are hollow in their interior and have an upper surface resembling that of a waffle to provide extended surface area for heat transfer purposes. The hollow plates 27 serve in the illustrated system as a reservoir for the freon that is being condensed. The undersurfaces of the condenser plates are covered with insulation 43 to provide a thermal barrier between them and the refrigeration compartments 17. To achieve the desired advantages from the V-shaped arrangement, each of the condenser plates 27 should be disposed at an angle between about and about 40 to the vertical. This arrangement has been found to both produce the desired flow-directing effect from the undersurface and to produce excellent heat-transfer on the interior plate surfaces as a result of the effect of gravity forcing the solid CO; nuggets downward against the plate surfaces and the particular angle of contact preventing bridging of the nuggets in the hopper.

The evaporator plates 31 each have a hollow center section 45 having both surfaces of generally wafile-like design. As seen in FIG. 2, a peripheral flange portion The freon liquid that condenses in the condensers 27 flows downward through a tee 49 into a line that contains a modulating valve 51. The liquid stream passing through the valve 51 flows downward through an insulated pipe 52 to a location near the bottom of the cart where it enters another tee member 53 and is split into a pair of lines 55 which lead to inlets at the respective bottoms of the evaporators 31. Because the tee member 53 is located at a position below the evaporator plates 31, the amount of liquid freon in both evaporators is equalized because the fluid interconnection equalizes the hydraulic head in both. In the evaporators 31, the cooled liquid freon slowly vaporizes, absorbing heat from the air adjacent the surfaces of the evaporator plates. If one of the refrigeration compartments should be subjected to a greater heat load than the other, automatic compensation is provided via the hydraulic head equalization. The warmer vapor exits from outlets at *thetop of the evaporator plates 31 and flows through conduits 57 which lead to inlets near the upper ends of the condenser plates 27. l-Ieat transfer from the warmer vapor to the cold solid carbon dioxide nuggets, which have a surface temperature of about 1l0 F., rapidly takes place and the freon vapor is reliquified.

In the illustrated embodiment, a temperature sensor 59 is located near the rear of the lefthand refrigeration compartment 17 which is connected to the modulating valve 51. In response to the temperature read by the sensor 59, the valve 51 either slightly opens or closes to adjust the rate of flow of freon through the valve downward through the line 52 and into the bottom of the evaporator plates 31 to cause the temperature of the compartments 17v to approach the desired temperature. The rate of flow through the valve 51 will likely I vary depending upon the temperature of the liquid leaving the condensers 27. It has been found that the angular disposition of the walls of the hopper 23 assures good heat transfer to the solid C0, nuggets even when the apparatus 11 has been in use for some time and less than half of the expendable CO, nuggets remain in the hopper. The angular disposition assures, with the assistance of gravity, the prevention of bridging of the nuggets in the hopper and provides good heat transfer near the bottom of the condenser plates 27 where the liquid freon is at its coolest before leaving the condensers for another circuit through the closed loop system.

It should be understood that the cantilever arms 21 are preferably relatively narrow in width and therefore leave ample space between the arms at each vertical level for the circulation of air in the refrigeration compartments 17 Of course, when food trays or the like are supported in the compartments, the region for circulation is somewhat restricted. However, the size of the compartments 17 are preferably proportioned to be substantially larger than the trays which will be accommodated therewithin so that there is accordingly space remaining for air circulation between the peripheries of the food trays and the walls of the enclosure 13. As a result, natural convection flow is created as the air is cooled adjacent the evaporator plates 31 and descends to the bottom of the cart chassis. This effect is particularly pronounced in the chimney where there is minimal outer wall surface for heat inflow and where the pair of spaced evaporator plates 31 define the major portion of the boundary. The cool descending column of air in the chimney flows through the lower openings 33 under the evaporator plates into the refrigerator compartments 17 where it displaces the air therein which is slowly being warmed. The heat flow into the cart 11 is through the walls of the enclosure and the upward convection currents are located generally about three sides of the tray peripheries adjacent the inner wall surfaces of the enclosure 13. The warm air enters the top of the chimney 29 through the upper openings 35 where it is smoothly directed downward by the undersurfaces of the hopper 23.

Depending upon a number of considerations, such as the type of material to be refrigerated, the temperature to be maintained in the refrigerator compartments 17, the desired time within which it is desired that equilibrium temperature should be reached after loading the hopper with CO nuggets and the environment within which the cart will be used, the amount of circulation that is achieved by natural convection may be sufficient. However, should a more rapid cool-down of material be desired, for example, or should greater uniformity of temperature be desired within the refrigeration compartments than is accomplished, the blower unit 37 is located in association with the central chimney. To retain the portability of the cart 11, the blower is preferably battery-powered. The blower 37 could be employed to merely assist the convection flow and thereby increase the draft down the chimney and out through the lower openings 33 into the bottom of the respective refrigeration compartments 17. However, it has been surprisingly found that even more effective cooling is achieved if the blower 37 is directed upwardly so as to reverse what would otherwise be the natural convection pattern within the enclosure 13. The upward moving column of cool air in the chimney 29 is split into two halves when it reaches the undersurfaces of the hopper 23, and the angular inclination of these surfaces effectively and efficiently directs the cooled air outward and across the upper portions of the refrigeration compartments 17. From these upper locations, the circulation is downward past the peripheries of the tray and then back into the chimney 29 through the lower openings 33 adjacent the location of the blower.

One example of a refrigeration cart 11 is constructed and operated which incorporates various features of the invention. The cart 11 incorporates two side-byside refrigeration compartments 17 each measuring approximately 22 inches wide by about 53 inches high by 25 inches deep. Each compartment 17 is adapted to hold ten food trays each of which is about 17.5 inches by 21 inches.

A V-shaped hopper 23 is provided having an upper insulated access door 25 in the top wall of the chassis 13, and approximately 20 pounds of solid CO nuggets are used to fill the hopper. The hopper 23 is defined by the upper surfaces of two condenser plates 27 each measuring approximately 11 inches by 19 inches and each having a waflle upper surface design. Suitable flanges at each end of the condensers 27 extend to the walls of the cart enclosure 13 and support the condensers by appropriate attachment thereto. Located below the condenser plates 27 are a pair of parallel evaporator plates 31, which are vertically disposed and are spaced approximately 7 inches apart. The evaporator plates 31 individually measure about 22 inches by 24 inches and have central sections 45 of wafile surface design about 20 inches by 22 inches in dimension.

A closed freon loop system is employed similar to that shown in FIGS. 1 and'2 of the drawings and contains approximately 3.5 lbs. of Freon-l2. A modulating valve 51 is employed which is regulated by a temperature sensor 59 located generally centrally of the lefthand refrigeration compartment 17 at a position approximately 15 inches below the top of the compartment and 10 inches forward of the rear wall 41. The temperature sensor 59 is suitably supported from the underside of one of the brackets 21 that support the food trays. One-half inch thick foamed styrene insulation is provided at the undersurfaces of each of the condenser plates 27.

The system is set to achieve a temperature of 40 plus or minus 5 F. in the refrigeration compartments 17. Starting with the cart 11 at an ambient temperature of F. and using only natural convection, the system cools to below 45 F. in twenty minutes and maintains this temperature for over 4 hours on the original load of CO nuggets. Starting from an ambient temperature of about F it is found that cool-down to or below 45 F. occurs in less than 30 minutes, which is considered to be excellent performance.

Shown in FIGS. 3 through 5 is a refrigerated shipping container 71 particularly designed for use in air freight transportation. Such a shipping container 71 is proportioned to fit into the cargo section of an airplane, and the illustrated container is particularly shaped for transportation in the belly section of the 8-747 airplane. In the cross-sectional view of the refrigerated container 71 shown in FIG. 3, it can be seen that the container is constructed with parallel top and bottom insulated walls 73,75, and a pair of parallel sidewalls 77,79. The left-hand sidewall 77 is shorter than the right-hand sidewall 79 and is connected via an inclined wall 81 to the bottom wall. This construction relieves the lower left-hand comer of the container 71 and adapts it to the available storage region in the curved body of the airframe.

As may be inferred from FIG. 4, the length of the refrigerated container 71 may be varied as desired and as convenient for whatever handling equipment will be employed to load and unload the refrigerated containers from the cargo space of the airplanes. Eachcontainer 71 also has a front wall 83 and a rear wall 85, both of which are insulated and extend completely across the front and rear of the containers, thus providing a completely enclosed and insulated compartment within the refrigeration container. Both the front and rear walls 83,85 may be attached by hinges to function as overall doors, or they may be affixed to the remainder of the container and provided with separate smaller doors. In the illustrated embodiment, an upper horizontal shelf 87 and a lower horizontal shelf 89 are provided to subdivide the container into three separate compartments which facilitate the orderly loading and placement of articles to be transported. The shelves 87,89 are suitably apertured to permit the convection flow of air vertically within the compartment. If additional access for the easy loading and unloading of the articles to be transported in the refrigerated container is desired, access doors (not shown) may also be provided in one or both of the sidewalls 77,79 of the container 71.

A hopper or reservoir 91 for holding the solid carbon dioxide refrigerant is located in a region having a generally triangular cross-section adjacent the lower left-hand comer of the container (as viewed in FIG. 3) which inherently would otherwise constitute the least desirable storage space within the container 71. An access door 93 is provided in the front wall 83 of the compartment to permit the loading of carbon dioxide nuggets into the hopper. Depending upon the length of the container 71, it may be desirable to employ a similar access door in the rear wall 85 of the container. Moreover, if desired, a trap door (not shown) might be provided in the lower shelf 89 at a location above the hopper which would be useful to fill the hopper with CO, nuggets prior to loading of the articles to be transported into the container.

In the illustrated container 71, a refrigeration system 95 is employed utilizing a closed freon loop employed to achieve a controlled transfer of the cooling capacity of the carbon dioxide nuggets in the hopper to a location adjacent the underside of the top wall 73 of the container, so that the air cooled at this upper location establishes a natural convection flow downward in the compartment which serves to force the warmer air upward where it in turn is cooled. The refrigeration system 95 employs a pair of condenser plates 97,99 which constitute the hopper 91. The condenser plate 97 is located at approximately a 45 angle adjacent the inner surface of the inclined wall 81 of the container, whereas the other plate 99 is disposed generally vertically, extending from the underside of the lower shelf 89 to the bottom 75 of the container. Both of the condenser plates 97,99 are generally coextensive with the length of the container, and short extension sections 101 are provided between the ends of the condenser plates and the front and rear walls.

The condenser plates 97,99 are hollow and are preferably constructed with their facing surfaces, which define the hopper 91, formed in a generally waffle-type pattern to provide extended surface area for heat-transfer purposes. In the illustrated embodiment, as best seen in FIG. 3, the two condenser plates 97,99 are joined at their lower edges so that the hollow internal chambers of both plates are in communication with each other along the entire length of the hopper 91. A single outlet 103 near the bottom of the condensers 97,99 is connected to an insulated riser 105 that leads upward to one end of a horizontal evaporator 107 which is located spaced slightly below the top wall 73 of the container 71 and is generally coextensive with the top wall. This evaporator 107 is likewise hollow and is preferably formed with an extensive surface area, for example a waflle-type pattern, on both surfaces to facilitate heat-transfer thereat.

An insulated return pipe 109 for returning the warmed freon, liquid or vapor, to the condensers 97,99 is located at the opposite end of the evaporator 107 near the rear thereof. As seen in FIGS. 3 and 4, the return pipe 109 leads downward generally adjacent the rear comer of the right sidewall 79 and then runs just under the lower shelf 89 to a tee member 111 which splits the flow and is connected to a pair of lines 113.

and 115 which respectively lead to the upper portions of the condenser plates 97 and 99. Control of the flow of freon in the system is achieved via modulating valve 117 which is disposed in the riser pipe 105 at a location just above the lower shelf 89. A temperature sensor 119 is located generally centrally within the compartment at a location just below the upper shelf 87 which effects control of the modulating valve.

In order to lift the cooled freon liquid from the condensers 97,99 which serve as a reservoir for the freon, a bubble-lift mechanism 121 is employed. As best seen in FIGS. 4 and 5, the bubble-lift mechanism 121 is located vertically above the modulating valve 1 17. The bubblelift mechanism 121 applies localized heat to the freon within the riser leading from the bottom of the condensers where it has been cooled. This application of heat causes localized boiling thus creating bubbles of freon vapor in the cooled stream which lift the accompanying liquid freon upward to the evaporator 107. This lifting effect is aided by closed loop refrigeration system 95 characteristics wherein the condensing of the freon vapor in the upper portions of the condenser plates 97,99 tends to create a suction effect in the return line 109 that facilitates the removal of the freon vapor from the opposite end of the evaporator 107.

In the bubble-lift mechanism 121 shown in FIGS. 4 and 5, a thin rod 123 having good thermal conductivity is located with a bent end portion 125 disposed coaxially within the riser pipe 105 above the modulating valve 117. The rod may be made of copper, stainless steel or aluminum, for example. The rod 123 enters through a hole in the sidewall of the riser 105 which is subsequently sealed. The rod 123 extends through the insulated sidewall 77 of the container and leads to a depression 127 formed in the outer skin of the wall. In the depression 127, radiator fins 129 are connected to the rod 123 to facilitate heat-transfer thereto.

The radiator fins 129 approach the temperature of the environment exterior of the container 71, and heat from the radiator fins is transferred via conduction to the bent end portion 125 of the rod that is disposed in the liquid freon stream in the riser 105. The relatively higher temperature of the end portion 125 of the rod causes localized boiling of the freon at the surfaces thereof, resulting in the formation of bubbles of freon vapor. These bubbles of vapor, because of their lighter density, create a lifting effect which carries the liquid freon with them upward to the evaporator 107. The location of the fins 129 in the depression 127 aids in preventing damage thereto during normal handling of the container 71.

It has been found that such a bubble-lift mechanism 21 is extremely effective in raising liquid freon from lower condensers 97,99 to an overhead evaporator 107. As seen in FIG. 5, the boiling adjacent the end portion 125 of the rod creates small bubbles which generally combine to form larger bubbles that actually span the entire internal diameter of the riser 105. It has been found that using an easily vaporizable heattransfer liquid, such as Freon-l2 (dichlorodifluoromethane), the bubble-lift can efficiently raise the cooled liquid freon as high as twelve feet in a riser having a five-eighths inch internal diameter. Generally, a riser employed will have an inner diameter between about one-quarter inch and fiveeighths inch, although other size pipes may be employed.

Using a pair of condenser plates 97,99 having a total surface area of about 110 square inches on the two facing surfaces that define the hopper 91 for the carbon dioxide nuggets, adequate circulation of the freon is obtained using the illustrated bubble-lift mechanism to insure a fairly unifonn temperature of 1* 5 F. within the refrigerated compartment when the temperature external of the compartment is between about 50 and 120 F., for example, with one setting of the modulating valve 117. By setting the modulating valve 117 differently, the internal temperature of the refrigerated compartment may be reasonably uniformly maintained at temperatures between about l5 and 50 F. For a system using a bubble-lift mechanism of this general type, a heat-exchange liquid is usually selected which has a vapor pressure at least about equal to that of Freonl 2, which has a vapor pressure of about one atm. at F for example. Liquids having lower vapor pressures may not provide adequate lifting power.

Shown in FIG. 6 is a bubble-lift mechanism 131 of an alternative design. The mechanism 131 includes an element 133 which is generally coaxial with the riser 105' but which, instead of being a solid rod, is a hollow tube the end of which is closed. The main portion of the tube 135 extends from the sidewall of the riser 105' through the skin of the container to a small bulb-type reservoir 137 which is located in a depression 138 in the sidewall of the container. This alternative design bubble-lift mechanism 131, instead of relying merely upon thermal conduction, as in the case of the mechanism 121 illustrated in FIGS. 4 and 5, supplies some heat via conduction through the sidewall of the tube 135 and uses convection to additionally supply heat to the coaxial element 133 along the surface of which boiling occurs.

A suitable vaporizable liquid 139, such as Freon-12, is sealed within the bulb 137 and tube 135. The liquid 139 is generally selected based upon the ambient temperature to which the exterior surface of the refrigerated container will usually be exposed and is chosen so that it boils below that temperature. As a result, boiling within the bulb 137 causes warm vapor to rise into the upper bent end portion 133 of the tube that is disposed within the riser 105'. Heat transfer takes place across the tube sidewall at the location of the end portion 133, causing the liquid freon in the riser to boil (creating bubbles for the lift effect) and simultaneously condensing vapor within the end portion of the tube. The condensed vapor runs down the internal surface of the tube back into the bulb 137 where it is subsequently again vaporized. It has been found that this inverse loop arrangement provides extremely efiective transfer of heat into the riser 105' at the location desired. In one example for use with a refrigeration system employing Freon-l2 as the heat exchange fluid, a copper tube having a one-fourth inch internal diameter is used, and the bulb is partially filled with Freon-l2 as the vaporizable liquid. Performance of this bubble-lift mechanism is excellent.

In addition to employing boiling of the heat exchange liquid to provide the desired bubble lift to raise the liquid from the bottom of the condensers to the overhead evaporator, the subliming carbon dioxide nuggets also provide a ready source of gaseous CO which may be utilized for this purpose. Shown in FIG. 7 is a hubble-lift mechanism 141 which employs the CO vapor for this function. A heat exchange liquid having a boiling point of at least about F. and preferably at least about 200 F. at atmospheric pressure, and a fairly low vapor pressure below 0 F. is preferably used in the refrigeration system. Of course, the freezing point should be below that of the solid CO refrigerant, i.e., about 1 10 F. One example is d-limonene' which boils at about 350 F. and has a very low vapor pressure at the temperatures of normal operation. The nugget hopper is made pressure-tight, and a tube 143 from the pressure-tight hopper is simply inserted into the riser 105" at a location above the modulating valve. Carbon dioxide vapor from the open end of the tube 143 forms bubbles in the heat exchange liquid which creates an upward current leading into the evaporator. In such a system, separation means would be likely employed in the upper surface of the evaporator to allow the carbon dioxide vapor to escape from the refrigeration loop after it had served its purpose of lifting the cooled heat exchange liquid upward to the overhead evaporator, and a check valve would be included in the tube 143. Moreover, vaporization of the heat exchange liquid would not occur in the evaporator, but the liquid would merely be warmed by heat transfer from the warmer air at the top of the compartment. The warm liquid would be forced out of the evaporator and downward through the return line to the upper portions of the condensers where it would be cooled by heat exchange with the subliming nuggets.

Various changes and modifications as would be obvious to one having the ordinary skill in the art may be made without deviating from the scope of the invention which is defined by the claims appended hereto.

Various of the features of the invention are set forth in the following claims.

What is claimed is:

1. Refrigeration apparatus utilizing solid carbon dioxide as a refrigerant and a circulating vaporizable heat-exchange liquid, which apparatus comprises means defining an insulated refrigeration compartment for maintaining temperatures therein below ambient, evaporator means disposed in the upper portion of said refrigeration compartment, hopper means for holding a quantity of solid carbon dioxide, hollow condenser means in association with said hopper means, said condenser means being located at a vertical level lower than said evaporator means, first conduit means connecting an inlet of said evaporator means to an outlet of said condenser means, second conduit means connecting an outlet of said evaporator means to an inlet of said condenser means, means for providing localized heat to the heat-exchange liquid from said condenser means to in said first conduit means to cause localized boiling and create bubbles of gas within said first conduit means for lifting the heat-exchange liquid from said condenser means to said evaporator means, valve means associated with said firstconduit means at a location between said condenser means and said bubblecreating means for controlling the flow of heatexchange liquid through said first conduit means, said temperature sensing means in association with said refrigeration compartment connected to said valve means for regulating said valve means to achieve a desired temperature in said refrigeration compartment.

2. Refrigeration apparatus in accordance with claim 1 wherein said bubble-providing means includes an element extending into said first conduit means which is maintained at a temperature sufficiently above the temperature of the heat-exchange liquid in said first conduit means to cause localized boiling thereof.

3. Refrigeration apparatus in accordance with claim 2 wherein said element is hollow and is connected with a source of a vaporizable liquid which boils at a temperature below the temperature exterior of said insulated refrigeration compartment.

4. Refrigeration apparatus in accordance with claim 2 wherein said element is made of said material and is provided with fins exposed to ambient temperature.

5. Refrigeration apparatus utilizing solid carbon dioxide as a refrigerant and a circulating heat-exchange liquid having a boiling point of at least about 150 F. at atmospheric pressure, which apparatus comprises means defining an insulated refrigeration compartment for maintaining temperatures therein below ambient, evaporator means disposed in the upper portion of said refrigerated compartment, hopper means for holding a quantity of solid carbon dioxide and for maintaining a pressure of CO vapor therein greater than atmospheric pressure, hollow condenser means in association with said hopper means, said condenser means being located at a vertical level lower than said evaporator means, first conduit means connecting an inlet of said evaporator means to an outlet of said condenser means, second conduit means connecting an outlet of said evaporator means to an inlet of said condenser means, means for injecting CO, vapor from said hopper means into said first conduit means to provide bubbles for lifting said heat-exchange liquid from said condenser means to said evaporator means, and separation means associated with said evaporator means for venting the CO vapor therefrom.

6. Refrigeration apparatus in accordance with claim 5 wherein valve means is associated with said first conduit means at a location upstream of said bubbleproviding means for controlling the flow of heatexchange liquid through said first conduit means, and wherein temperature sensing mans is associated with said refrigeration compartment and connected to said valve means for regulating said valve means to achieve a desired temperature in said refrigeration compartlf in a method of refrigeration utilizing solid carbon dioxide as a refrigerant and a vaporizable heatexchange fluid to cool an insulated refrigeration compartment to a temperature below ambient, which method includes cooling the heat-exchange fluid in a condenser using solid carbon dioxide, circulating said cooled heat-exchange fluid upward to evaporator means disposed within an upper portion of said refrigerated compartment at a vertical level above the condenser, absorbing heat in the heat-exchange fluid in the evaporator means to thereby cool the refrigeration compartment and returning said heat-exchange fluid from the evaporator means to the condenser, the improvement which comprises applying localized heat to the cooled heat exchange fluid in a conduit leading upward to the evaporator means from the condenser to thereby cause localized boiling which creates sufficient bubbles of vapor in the heat-exchange fluid to lift the fluid to the upper evaporator means and wherein the temperature within the insulated compartment is sensed and the flow of heat-exchange fluid from the condenser to the evaporator means is controlled in response to said temperature sensed by opening and closing valve means located between the condenser outlet and the point of localized heat application.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 3,695,056 Dated October 3, 1972 Inventor(s) Emmett P. Glvnn and Howard L. Hsu

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 2, line 39 change "34" to 35.

C01. 2, line.64, change "convention" to convection--.

Col. 9, line 6, change "21" to l2l-.

Col. ll, lines 7-8, delete "frorn saidc ondenser means to" C01. ll, line 15, change "said" (second occurrence) to --and.

Col. 11, line 32, change "said" (second occurrence) to -,-heat-conducting.

Col. 12, line 16, "mans should be means-.

Signed and sealed this 6th day of March 1973.

(SEAL) Attest:

EDWARD M. FLETCHER,JR. ROBERT GOTTSCHALK Attcsting Officer Commissioner of Patents FORM 0-1050 (10-69) USCOMM DC 603764369 n u.s. covznumzn-r PRINTING OFFICE I969 o-aas-a:4. 

1. Refrigeration apparatus utilizing solid carbon dioxide as a refrigerant and a circulating vaporizable heat-exchange liquid, which apparatus comprises means defining an insulated refrigeration compartment for maintaining temperatures therein below ambient, evaporator means disposed in the upper portion of said refrigeration compartment, hopper means for holding a quantity of solid carbon dioxide, hollow condenser means in association with said hopper means, said condenser means being located at a vertical level lower than said evaporator means, first conduit means connecting an inlet of said evaporator means to an outlet of said condenser means, second conduit means connecting an outlet of said evaporator means to an inlet of said condenser means, means for providing localized heat to the heatexchange liquid from said condenser means to in said first conduit means to cause localized boiling and create bubbles of gas within said first conduit means For lifting the heat-exchange liquid from said condenser means to said evaporator means, valve means associated with said first conduit means at a location between said condenser means and said bubble-creating means for controlling the flow of heat-exchange liquid through said first conduit means, said temperature sensing means in association with said refrigeration compartment connected to said valve means for regulating said valve means to achieve a desired temperature in said refrigeration compartment.
 2. Refrigeration apparatus in accordance with claim 1 wherein said bubble-providing means includes an element extending into said first conduit means which is maintained at a temperature sufficiently above the temperature of the heat-exchange liquid in said first conduit means to cause localized boiling thereof.
 3. Refrigeration apparatus in accordance with claim 2 wherein said element is hollow and is connected with a source of a vaporizable liquid which boils at a temperature below the temperature exterior of said insulated refrigeration compartment.
 4. Refrigeration apparatus in accordance with claim 2 wherein said element is made of said material and is provided with fins exposed to ambient temperature.
 5. Refrigeration apparatus utilizing solid carbon dioxide as a refrigerant and a circulating heat-exchange liquid having a boiling point of at least about 150* F. at atmospheric pressure, which apparatus comprises means defining an insulated refrigeration compartment for maintaining temperatures therein below ambient, evaporator means disposed in the upper portion of said refrigerated compartment, hopper means for holding a quantity of solid carbon dioxide and for maintaining a pressure of CO2 vapor therein greater than atmospheric pressure, hollow condenser means in association with said hopper means, said condenser means being located at a vertical level lower than said evaporator means, first conduit means connecting an inlet of said evaporator means to an outlet of said condenser means, second conduit means connecting an outlet of said evaporator means to an inlet of said condenser means, means for injecting CO2 vapor from said hopper means into said first conduit means to provide bubbles for lifting said heat-exchange liquid from said condenser means to said evaporator means, and separation means associated with said evaporator means for venting the CO2 vapor therefrom.
 6. Refrigeration apparatus in accordance with claim 5 wherein valve means is associated with said first conduit means at a location upstream of said bubble-providing means for controlling the flow of heat-exchange liquid through said first conduit means, and wherein temperature sensing mans is associated with said refrigeration compartment and connected to said valve means for regulating said valve means to achieve a desired temperature in said refrigeration compartment.
 7. In a method of refrigeration utilizing solid carbon dioxide as a refrigerant and a vaporizable heat-exchange fluid to cool an insulated refrigeration compartment to a temperature below ambient, which method includes cooling the heat-exchange fluid in a condenser using solid carbon dioxide, circulating said cooled heat-exchange fluid upward to evaporator means disposed within an upper portion of said refrigerated compartment at a vertical level above the condenser, absorbing heat in the heat-exchange fluid in the evaporator means to thereby cool the refrigeration compartment and returning said heat-exchange fluid from the evaporator means to the condenser, the improvement which comprises applying localized heat to the cooled heat exchange fluid in a conduit leading upward to the evaporator means from the condenser to thereby cause localized boiling which creates sufficient bubbles of vapor in the heat-exchange fluid to lift the fluid to the upper evaporator means and wherein the temperature within the insulated compartment is sensed and the flow of heat-exchange fluid from the conDenser to the evaporator means is controlled in response to said temperature sensed by opening and closing valve means located between the condenser outlet and the point of localized heat application. 